1. Field of the Invention
The present invention relates to atomic stabilized frequency sources and more particularly, to an improved RF source circuit for igniting the discharge in a ribidium lamp of a rubidium frequency standard.
2. Prior Art
Although the present invention may find practical application in any one of numerous atomic stabilized frequency sources, it is particularly adaptable for operation in a rubidium vapor cell frequency standard. Rubidium vapor cell frequency standards, as well as other types of atomic stabilized frequency sources, are described extensively in the literature. For example, reference may be had to the texts respectively entitled, "Frequency and Time" by P. Kartaschoff, Academic Press, 1978; and "Frequency Synthesizers Theory and Design", Second Edition, by Vadim Manassewitsch, John Wiley & Sons, 1980. Such frequency sources are stabilized by quantum mechanical atomic state transition resonances such as the hyperfine atomic resonance frequency related to the change in the internal energy of the atom. A rubidium frequency standard operates as a discriminator based upon the energy absorption characteristics of rubidium-87. In practice, a rubidium lamp passes a lightbeam into a rubidium absorption cell. The rubidium cell absorbs some of the light energy because of the energy level transitions in the rubidium-87 gas. When an electromagnetic field frequency equal to the resonance frequency of the rubidium vapor is applied to the vapor cell, the number of energy level transitions in the rubidium-87 gas is increased and more of the light emitted by the rubidium lamp is absorbed by the rubidium vapor cell. Typically, a photodiode is used to detect the occurrence of the maximum absorption of light from the rubidium lamp which occurs when the frequency of the excitation electromagnetic field exactly matches the rubidium resonance frequency. Typically, a frequency synthesizer is used to generate the appropriate electromagnetic field frequency of approximately 6,834.685 MHz. This field frequency is modulated at a relatively slow rate (i.e., 154 Hz.) so that the photodiode provides a demodulated signal which may be applied to a phase detector or comparator which also receives a reference modulation signal. The output of the phase comparator is a DC error voltage which is used to control a voltage controlled crystal oscillator at a selected frequency, typically of 5 or 10 MHz. In this manner, the frequency of the crystal oscillator is stabilized to approximately one part in 10.sup.11 or better over long periods of time to provide a highly stable and accurate frequency source.
Typically, because of the chemical characteristics of the rubidium gas and the requirement for a stable and selected light intensity and spectrum, the rubidium lamp is heated to a selected operating temperature and is ignited by an electrodeless RF field excitation generated by means of a coil driven by an RF source. Unfortunately, the impedance of the lamp traverses extremes, equivalent to a pure capacitance prior to ignition and becoming a complex impedance changing with the level of the RF ignition signal and the lamp temperature. This impedance change occurs because the carrier gas used with the rubidium within the lamp container ignites at a lower potential than the rubidium. As the lamp heats up the rubidium vapor enters an ionized state eventually suppressing the carrier gas light output. In the prior art an RF oscillator is used which can withstand the impedance variation while delivering power to the lamp since a fixed matching circuit to match the changing impedance is impractical. The lamp then becomes part of the oscillator circuit by field coupling causing the frequency of the RF excitation field to change with the lamp impedance. Unfortunately, such RF oscillator frequency changes tend to destabilize the lamp light output producing instability in the frequency of the rubidium source. Furthermore, after the lamp has been ignited and stabilized, it is not uncommon for a momentary power interruption to cause the lamp to reignite in an undesirable mode or even worse, to fail to reignite at all. This, of course, has the effect of precluding proper operation of the rubidium frequency standard.